I'd keep my money. Vista doesn't really offer any new functionality other than application muting (something OS X really needs BTW) and better drive-indexing. Other than that, it consumes more resources.
It's not a bad OS or anything, it just doesn't offer anything different to XP so i'd save my money/cpu cycles and stick with XP.
On a side note (as magicmoxter recommended) W7 is shaping up quite well, so it's probably a good idea to hold off until that comes around as it's pretty much MS's answer to SL in that it is reducing footprint and optimizing performance on modern and legacy hardware alike.
I'm running the beta now and it's not as staggeringly wonderful as many would have you believe but it's definitely a step in the right direction which something MS have needed to make for a while now..